The present invention relates to the field of microwave thermal therapy of tissue. In particular, the present invention relates to a catheter for transurethral microwave thermal therapy of benign prostatic hyperplasia (BPH).
The prostate gland is a complex, chestnut-shaped organ which encircles the urethra immediately below the bladder. This relatively small organ, which is the most frequently diseased of all internal organs, is the site of a common affliction among older men, benign prostatic hyperplasia (BPH), as well as a more serious affliction, cancer. BPH is a nonmalignant, bilateral nodular tumorous expansion of prostate tissue occurring mainly in the transition zone of the prostate. Left untreated, BPH causes obstruction of the urethra which usually results in increased urinary frequency, urgency, incontinence, nocturia and slow or interrupted urinary stream. BPH may also result in more severe complications, such as urinary tract infection, acute urinary retention, hydronephrosis and uraemia.
A fairly recent treatment method for BPH involves microwave thermal therapy, in which microwave energy is employed to elevate the temperature of tissue surrounding the prostatic urethra above about 45.degree. C., thereby thermally damaging the tumorous BPH tissue. Delivery of microwave energy to tumorous prostatic tissue is generally accomplished by a microwave antenna-containing applicator, which is positioned within a body cavity adjacent the prostate gland. The microwave antenna, when energized, heats adjacent tissue due to molecular excitation and generates a cylindrically symmetrical radiation pattern which encompasses and necroses the tumorous prostatic tissue. The necrosed intraprostatic tissue is subsequently reabsorbed by the body, thereby relieving an individual from the symptoms of BPH.
A safer and more effective treatment of BPH is transurethral microwave thermal therapy. This method of treatment minimizes the distance between a microwave antenna-containing applicator and the transition zone of the prostate by positioning a Foley-type catheter-bearing applicator adjacent to the prostate gland within the urethra. Due to the close proximity of the microwave antenna to the prostate, a lesser volume of tissue is exposed to the cylindrically symmetrical radiation pattern generated by the microwave antenna, thereby minimizing the amount of healthy tissue necrosed. Intraurethral applicators of the type described can be found in Turner et al. U.S. Pat. No. 4,967,765 and Hascoet et al. European Patent Application 89403199.6.
Recent improvements in transurethral thermal therapy catheter design have resulted in even more effective application of microwave radiation applied to prostatic tissue. For instance, recent transurethral catheters such as that described in Rudie U.S. Pat. No. 5,413,588, issued May 9, 1995, include shafts having a multiplicity of lumens arranged about a lumen carrying a microwave antenna. The antenna lumen is oriented nearer a first side of the catheter shaft than a second side of the catheter shaft to position the microwave radiation closer to the first side of the catheter. Cooling lumens are arranged about the microwave antenna lumen to absorb a portion of the microwave radiation so that a greater amount of microwave radiation is absorbed on a second side of the catheter shaft than the first side. This arrangement creates an asymmetrical microwave radiation pattern to permit focusing a greater amount of microwave radiation toward a selected tissue, such as prostatic tissue anterior and lateral to the urethra. This transurethral catheter design also includes a lumen to facilitate urinary drainage from the bladder through the urethra during a treatment session.
Antennas which have been used for hyperthermia have a variety of inadequacies which preclude their application to microwave thermal therapy. First, such antennas often generate heat in two forms: microwave energy and heat energy due to resistive losses of the antenna. The efficiency of these antennas has not been of much concern due to the relatively low amount of energy used to generate interstitial temperatures of between about 43.degree. C. to 45.degree. C. and the lack of any adverse effect these temperatures had on healthy tissue. Furthermore, it is known in the art that the shape and size of a radiation pattern generated by some microwave antennas are in part a function of how deeply the antenna is inserted into the tissue. Prior microwave dipole antennas used for hyperthermia have been unable to provide a predictable heating pattern within tissue due to the variable effects caused by the depths of insertion of the antenna into the tissue. Finally, the radiation length of these antennas has not been easily variable to accommodate the varying sizes of prostates requiring treatment. The antenna designs of the prior art relating to hyperthermia, therefore, have proven unsatisfactory for microwave thermal therapy and its attendant higher temperatures.
The objective of microwave thermal therapy is to reduce the length of a treatment session and to selectively heat and necrose only undesirous tissue, while sparing, to the greatest extent possible, adjacent healthy tissue. In order to avoid damage to tissues immediately adjacent the microwave antenna containing applicator (i.e., the urethra, the ejaculatory duct and the rectum), it is essential that the resistive losses of the antenna he reduced or optimally eliminated. The ability to eliminate resistive losses and utilize only microwave energy to heat a targeted tissue area will permit a cooling system, such as that described in co-owned application Ser. No. 07/847,718, now U.S. Pat. No. 5,413,588, to maintain safe temperatures adjacent to the applicator by absorbing and carrying away any excess heat conducted to the urethral tissues. In addition, the ability to construct an antenna capable of producing a predictable, yet selectively variable size heating pattern would aid in achieving an effective treatment of undesirous tissue while minimizing harm to healthy tissue.